So I had some problems with my Yanmar 2GM20F today. I actually first noticed it the last time I took the boat out. The water coming out of the exhaust is PITCH black. Looks like dirty oil, but no slick. I changed the oil last weekend and the oil level has not changed. So I was out today and I was motor-sailing with the engine turning around 2500rpm when I noticed it again. I went down to idle and it goes clear again. Then (at the worst possible moment) the engine started running funny and was only turning 1200rpm at the same throttle position. I tired to throttle up and it bogged down. Ended up shutting it down and sailing home. Additionally, it has been a little hard to start. I dont have anything to base this on as it is a new boat to me and my first diesel but it just seems longer than I would expect, especially when the engine is cold..

Here is what I am thinking:

I think this is a fuel issue. The PO said that he never let it get down to 1/2 a tank, and would always keep it topped off. I now think that he may have been doing this for a reason. (I am currently at just under a 1/2 tank (was going to fill up today but never made it =))

I also thought about draining the tank and will probably still do that if this works.

The plan:

Going to change the fuel filters tomorrow and fill up with fresh diesel. See what happens

Does this sound like I am in the ballpark or am I totally off base. I am kinda new to diesels. If that doesn't do the trick where do I look next, etc.

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Telstar 28
New England

You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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Black exhaust could be soot as SD mentions. If you want to to determine if this is the culprit for the black water exhaust, disconnect the exhaust hose from the muffler and put your finger in it and see if the water in the muffler is black and see if the exhuast hose has any build up in it. If the exhaust water is black, disconnect the exhaust hose from the motor to the muffler to see if that is full of exhaust soot. If so, replace.

The sluggishness on idle/throttle could be related to the black water, but if you have good flow of water out the back, it probably isn't because a significant build up of of soot in the hoses, would result in reduced water flow and water backing up into the motor.

If you haven't changed the fuel filters since you have owned the boat, this could be the cause. A fuel tank that is 1/2 full or less isn't the cause. Do you have a water separator? If so, does it have a clear bulb on the bottom? What color is the fuel in it? If it isn't clear/pale yellow, that could be an indicator of dirty fuel. Changing both the primary and seconday fuel filters could make the difficult starting and the sluggish starting a thing of the past.

I'd say always start the investigation changing the least expensive items first (fuel and filters). Although diesel is getting almost as expensive as stainless steal (purposeful "a"). But I digress.

How often were you running the motor? Diesel engines do not like to sit for long periods, and while running, like to work hard. If she sat in one place for a long period, or was only used to get out of the marina, then you may have a motor that is protesting lack of use. But, before you go there, you may have water in the tank and fuel lines, and bad filters. Definitely drain everything and replace all the filters. If it still occurs, either buy Nigel Calders diesel book, or rely on some of the very knowledgeable diesel motor folk on this board.

I am assuming from the port on the stern of your boat you just came out of winter lay up? Checking the filters and exhaust elbow and pipe are both good suggestions. The exhaust elbow will have an area were the water comes in and mixes with the gases, the bend tends to get a build up of soot that severely restricts the exhaust flow. Also look closely at the water area as if it is corroded out it can allow water back into the enigne. Don't remove this until you have a new gasket in hand to reinstall it if it is ok. Also the previous owners warning about not going below 1/2 tank my tell the tale as well. The new low sulfur diesel fuel tends to grow more bacteria than the old stuff. So you may have water and or bacteria or other nasty stuff in the fuel that doesn't burn so well. When you look at your racor is there black stringy stuff in the fuel? If this is the case you may have to have your fuel polished and or the tank cleaned. The process involves pumping all the fuell out of the tank filtering it and then refiltering to clean it up. You can also buy some additives to help kill/fight the bacteria if this turns out to be the case. Start with the simple inexpensive stuff and work up.

Sounds like a fuel issue. I agree with starting by changing the filters. If they are very dirty and the filter bowl shows signs of water, then changing the fuel is probably next. If the fuel is old and never had stabilizer added, that may do it. If this doesn't fix things, you might need to have the injectors rebuilt.

Yanmars are hard to start in cold weather since they don't have glow plugs, so that may not be part of the problem.

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